Silicon halide compounds suitably used as a material for formation of silicon nitride film, undergo hydrolysis by mere contact with a slight amount of water vapor contained in the air, to form hydrogen chloride gas and a silanol group. It is known that the thus-formed silanol group gives an adverse effect on the property of the silicon nitride film formed from a silicon halide compound. Therefore, it is necessary to quantitatively determine the silanol group of a silicon halide compound for quality control of the compound.
In order to quantitatively determine a very small amount of the silanol group present in a silicon compound, infrared spectrophotometry is known. For example, in JP-A-9-318525 is described an infrared spectrophotometry wherein an optical path length, i.e. a length of sample layer through which an infrared light is transmitted, is set at 50 to 150 mm in order to measure a silanol group of about 0.1 ppm. In the method for measurement of silanol group, described in the literature, there is used a cell constituted by a stainless steel-made cylinder and infrared-transmitting, calcium fluoride-made windows fitted to the cylinder.
In order to measure a silanol group of a very small amount, for example, 0.1 ppm or less, it is generally necessary to remove a very small amount of the water adhering to a cell to be used for measurement and then place a sample into the cell. It is because, when a sample is placed in a cell of insufficient water removal and is measured for the concentration of silanol group, the residual water in the cell and silicon halide compound react with each other to form a silanol group, which is included in the measurement data obtained, as an error. Hence, in the method for measurement of silanol group, described in the JP-A-9-318525, it was necessary to beforehand pass a large amount of nitrogen gas of water content of 0.5 ppm or less, through the cell inside, or, in addition with the passing of the above mentioned nitrogen gas, to clean the cell inside with the sample per se. When the sample is a silicon halide of low volatility, such as hexachlorodisilane or the like, the above-mentioned cell cleaning has had to be conducted taking a very long time when a new sample is filled in the cell in place of the used sample.
As described above, with the conventional method, much time and labor have been required for the removal of water from cell inside, making difficult the rapid measurement of a very small amount of silanol group.
The present invention is intended to provide a method which can measure a silanol group of very small amount (0.05 to 0.1 ppm level) rapidly and easily, and a cell for measurement of infrared absorption spectrum, used in the method.